The proposed research will be an investigation of the mechanism of Na ion transport across rabbit urinary bladder. Conventional, ion specific microelectrodes and impedance analysis will be used to measure apical and basolateral membrane potentials, conductances and areas as well as intracellular ion activities at various Na ion transport rates of the urinary bladder. Measurement of apical membrane potential, conductance and current-voltage relationships while varying external ion composition in control and after hormonal stimulation of Na ion transport will allow modeling of the permeability properties of this membrane. The intracellular ion specific microelectrodes will facilitate the calculation of the passive permeabilities as well as yield insight into the mechanism of Na ion entry across the apical membrane and extrusion across the basolateral membrane. These experiments will also allow a determination of the ionic mediator for the negative feedback system between intracellular Na ion and apical membrane conductance. Gramicidin D will be used to reduce the apical membrane as a barrier between solution and cell cytoplasm. By using ion specific microelectrodes in conjunction with gramicidin D, the active properties of the Na ion - K ion pump will be investigated. This in situ activity will be compared to the microsomal activity of the Na ion - K ion pump. The results will contribute to the understanding of the cellular mechanisms of Na ion transport across tight epithelia.